The present invention relates to a method for reducing or completely eliminating water influx in an underground formation which contains hydrocarbons to the production borehole. The present invention furthermore relates to crosslinkable copolymers which can be used in the method. By means of this method, the blockage of water is achieved without thereby making it more difficult for oil and/or hydrocarbon gas to gain access to the borehole.
Water often exists as salt solution in the same formation as oil or gas. The recovery of oil or of hydrocarbon gas therefore entails the recovery of water in an amount such that it gives rise to considerable problems; it directly or indirectly causes deposition of salts in the vicinity of the borehole or in the borehole itself, it considerably increases the corrosion of all metal parts underground or above-ground, it increases the amounts of the pumped, transferred and stored liquids without benefits and, together with the oil, it creates emulsions which are difficult to break above-ground and which may form blockages underground in the cavities of the formation.
According to the prior art, numerous methods which are intended for reducing the water influx into the boreholes for the recovery of oil or hydrocarbon gas have been proposed and practiced. They often consist in introducing an impenetrable barrier in the formation between the water and the borehole or between the water and the oil or hydrocarbon gas. The means usually introduced block virtually just as much oil or hydrocarbon gas as water. The components of this barrier may be: cement, resins, suspensions of solid parts, paraffins or water-soluble polymers which are crosslinked by introduction of so-called crosslinking agents in the deposit.
At present, polymers are used which are introduced in solution into the porous medium, are adsorbed at the surface of the solid and penetrate into the pore space, so that they are suitable for reducing the water influx. In contrast, the nonaqueous fluids, such as oil or especially hydrocarbon gas, pass through the adsorbed macromolecules, which now occupy a negligible volume on the wall and thus leave the passage substantially unobstructed.
U.S. Pat. No. 4,095,651 discloses the use of hydrolyzed polyacrylamides. However, it has been found that this type of polymer is effective mainly against water having a low salt content and is rendered ineffective by water having a higher salt content. At relatively high temperatures, in the presence of polyvalent ions, these polymers tend to form precipitates which may block the pores of the rock formations.
U.S. Pat. No. 4,718,491 discloses the use of polysaccharides. Although these compounds, which are poorly injectable into the pore space, result in a delay or reduction of the water influx, they permit only incomplete utilization of the existing hydrocarbon reserves of the deposits or lose their activity at relatively high temperatures.
U.S. Pat. No. 4,842,071 discloses the use of unhydrolyzed acrylamide polymers or acrylamide copolymers, which are hydrolyzed by subsequent introduction of an aqueous basic solution. This method has disadvantages with regard to an additional effort due to the introduction of a further solution, and due to the problems of the accessibility of the injected polymer solution as a result of the subsequent placement of the basic solution and with regard to an increased corrosion susceptibility of the apparatuses used. Moreover, the polymer solution is effective only after reaction with the aqueous basic solution, the degree of effectiveness being determined by the degree of reaction.
EP-B-0 577 931 discloses a method for water blockage which makes use of polymers of 5–90% by weight of AMPS, from 5 to 95% by weight of N-vinylamides and, if required, up to 90% by weight of N,N-diallylammonium compounds and, if required, up to 90% by weight of a further olefinically unsaturated monomer. These polymers are uncrosslinked. This method is effective only in the case of relatively low permeabilities, for example in the case of gas probes having permeabilities in the region of a few mD (millidarcy).
WO-01/49971 discloses copolymers and a method for water blockage using these copolymers, which contain structural units of vinylphosphonic acid, acrylamide and, if required, also AMPS and N-vinylformamide, and which may be crosslinked with zirconium compounds. The amount of the crosslinkable phosphonic acid and carboxyl groups must be from 0.01 to 7.5 mol %.